When designing an RFID network, there are times when you have to trigger the readers to read at only certain times. To reduce interference and RF noise in an environment with more than one RFID reader, you can activate the interrogation zone only when needed. There are several ways to trigger interrogations. In addition to manual triggering (rarely used, because it requires a human), timed triggering, or software triggering, a common method is to use triggering hardware, such as motion sensors.
Motion sensors can function on several principles. A common kind of motion sensor, also called "photo eye," uses an infrared or laser beam. The transmitter sends the beam across the product's path, and as the product interrupts the beam, the sensor on the other side of the path (the receiver) registers the absence of the beam and sends a signal to the interrogation zone (mostly through middleware but sometimes directly) to start interrogation in a certain time interval. This type of motion sensor often is used with intrusion alarms; in RFID systems they are often used on conveyors.
Do not forget that you need two sides in order for a photo eye sensor to work. You will need to install it on opposite sides of a conveyor belt before the interrogation zone.
Motion sensors also can work on a "radar" principle: the sensor transmits RF waves to the environment and registers the waves reflected by a passing object. You can see these sensors in stores in conjunction with automatic door openers. Proximity sensors work on a similar principle, but the reflected waves and their properties are used for calculating the object's distance from the sensor.